1. Field of the Invention
The present invention relates to a temperature control of a lamp annealing furnace (optical heating apparatus), which is used for activating an ion implantation layer; reflowing phosphorus silicate glass; annealing metal; forming an ohmic contact between Si and metal; and the like during processing of a semiconductor wafer.
2. Description of the Related Art
A conventional lamp annealing furnace uses a process tube made of quartz glass. A semiconductor wafer to be subjected to heat treatment is placed inside the process tube and is heat-treated using light emitted from lamps (such as tungsten-halogen lamps) provided in the lamp annealing furnace. The lamps are grouped into two sets that are respectively arranged so as to be spaced apart from an upper surface and a lower surface of the process tube or to face a right surface and a left surface thereof.
Various techniques have been conventionally used to improve uniformity of temperature distribution in a wafer, which is under heat-treated. For instance, the lamps are arranged in a lattice manner or in a honeycomb manner. Also, a lamp power of each lamp is controlled or an area in which the lamps are arranged is divided into several zones and the lamp power control is performed with respect to each zone. Further, lamps having different wattages are used as the lamps of the lamp annealing furnace.
When the temperature in the lamp annealing furnace is controlled using these conventional techniques, if the transmittance of the quartz glass tube stays constant, the distribution of heat supplied from the lamps to a wafer may be reproducible. As a result, the temperature distribution of a wafer that is being processed is also reproducible. However, impurities emitted from a wafer during the heat treatment cause a clouding of the quartz glass tube. In particular, the reflow of phosphorus silicate glass causes the conspicuous clouding of the quartz glass tube because the phosphorus adheres to the surface of the quartz glass tube. Clouding of the quartz glass does not evenly occur and gets worse across the entire surface of the quartz glass tube, so that the distribution of heat supplied from the lamps to a wafer varies with time and the uniformity of the temperature distribution in the wafer is reduced.
To solve this problem, there have been conventionally used various techniques. With a first conventional technique, changing of a quartz glass tube is periodically performed. With a second conventional technique, a plate made of quartz glass is inserted into a space between an inner surface of a quartz glass tube and a wafer to be processed, and changing of such a quartz glass plate is periodically performed. With a third conventional technique, a quartz glass tube having transparency is changed into a frosted glass tube in order to suppress changes in transmittance thereof due to the clouding. Even with these techniques, however, it is impossible to prevent the reduction in uniformity of temperature distribution in a wafer caused by the clouding of a quartz glass tube.
According to the present invention, a transmission type sensor including a light emitting section and a light receiving section is arranged so that a quartz glass tube, in which heat treatment of a wafer is performed, is sandwiched between the light emitting section and the light receiving section. The transmission type sensor detects a change in the intensity of light that is emitted from the light emitting section, passes through the quartz glass tube, and is received by the light receiving section. A degree of change in the light intensity detected by the transmission type sensor is measured and a light transmittance of the quartz glass tube is determined using the measured degree of change in the light intensity. By using a plurality of transmission type sensors and by controlling balance among output power of lamps according to the light transmittance of the quartz chamber determined based on the detection result of each transmission type sensor, it becomes possible to improve the uniformity of temperature distribution in a wafer during heat treatment.